Polycarbonates represent a widely used class of materials which possess highly desirable physical properties relating to, for example, transparency, heat distortion resistance, toughness, and electrical resistance. These properties make the polycarbonates especially well suited for applications relating to, for example, molded products, extruded film, structural parts, tubing and piping, and prosthetic devices.
Polycarbonates generally encompass aliphatic and aromatic polycarbonates, with the aromatic polycarbonates being particularly desirable in commercial applications. Traditional methods for forming aliphatic polycarbonates have focused on the transesterification of diols with lower dialkyl carbonates, dioxolanones, or diphenyl carbonates in the presence of catalysts containing, for example, alkali metal, tin, and titanium compounds. Aliphatic polycarbonates have also been formed using zinc initiating species which copolymerize carbon dioxide and oxiranes.
Aromatic polycarbonates differ from aliphatic polycarbonates in that they typically possess a higher level of thermal stability. In general, these materials are synthesized by different methods. Typically, aromatic polycarbonates are prepared by reacting bisphenols with carbonic acid derivatives at high temperatures, e.g., by reacting a diphenol and a diphenyl carbonate. These reactions are usually carried out at low pressures (e.g., vacuum conditions) and high temperatures. Although these reactions may occur in the absence of organic solvents, the reactions are potentially disadvantageous in that complex apparatuses are often necessary, primarily due to the presence of a vacuum and the formation of a high viscosity of the melt polymer reaction product.
Solid state polymerization ("SSP") techniques are significant in that they are primarily used to chain extend low molecular weight polycarbonates to form higher molecular weight polymers. In order to carry out an SSP, it is often necessary to crystallize the polycarbonates. The polycarbonates are typically exposed to organic solvents such as methylene chloride, chlorobenzene, or chloroform to induce crystallization. The use of such organic solvents may be disadvantageous in that it raises certain potential environmental risks.
It is therefore an object of the present invention to provide processes of forming polycarbonates which do not require organic solvents and are not carried out under vacuum or low pressure conditions.